In a simple definition, in an electrical power transmission line, the maximum loading at which the inductive reactance becomes equal to the capacitive reactance and the surge impedance is completely terminated or becomes zero, that is called Surge Impedance Loading(SIL)

If you want to better understand this concept, then you must know what is surge impedance. Let's know,

When electrical power is transmitted over a long distance(more than 250 KM) through overhead transmission lines, capacitance and inductance effect occurs in those transmission lines. The capacitance effect mainly occurs between two live conductors. Those two live conductors act as electrodes and the air gap between those two conductors acts as a dielectric medium. And the inductance effect occurs due to the flow of high-value alternating current through the conductor which causes to create a magnetic flux around the conductor.

These inductance and capacitance effects have reactance properties such as inductive reactance and capacitive reactance respectively. And the difference between the inductive reactance and capacitive reactance of the transmission is known as Surge Impedance.

So, the surge impedance of the transmission line is given by inductive reactance and capacitive reactance of the line.

Actually, the surge impedance term is related to the surge voltage that occurs due to lightning or switching. The surge impedance is also known as characteristic impedance or natural impedance as it is created naturally and independent of the load.

1. Surge impedance indicates the capacitive and inductive of a transmission line.

2. Calculating the Surge impedance loading helps to find out the maximum load capacity of a transmission line in MW.

3. Surge impedance helps to find out the receiving end voltage with respect to the sending end voltage.

5. Surge impedance helps to understand the voltage regulation in a transmission line.

6. Calculating the surge impedance loading helps to find out the loss in a transmission line.

7. Calculating the surge impedance loading helps to find out the appropriate value of the shunt capacitor or series reactor to minimize loss and increase the efficiency of the transmission line.

Surge impedance loading can be called as a uniform load because it causes to flow of uniform current and voltage, it makes the system pure resistive, it makes the power factor unity.

At the surge impedance loading, capacitance creates reactive power and the inductance absorbed the reactive power. So, we can say the effective reactive power is zero. In this case, the transmission line is assumed as lossless so the characteristics impedance(Zc) = Surge Impedance(Zs). The unit of surge impedance loading is watt or megawatt.

At the surge impedance loading, the receiving end voltage is equal to the sending end voltage. Also, the whole system act as a pure resistive. Surge impedance is directly proportional to the inductance and inversely proportional to the capacitance. So the value of surge impedance can be reduced either by increasing the capacitance of the line or by decreasing the inductance of the line.